Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by progressive cognitive decline and dementia. Recent genome scans have identified over twenty novel AD susceptibility loci, and several of these loci implicate the immune system. Our recent gene expression analyses of data from healthy young individuals have implicated 12 AD susceptibility genes in myeloid cell function, whose expression, relative to each risk allele, is altered in the primary monocytes. We have also recently identified AD risk-increasing alleles of the myeloid cell surface receptor CD33 that are associated with diminished A? uptake by human monocytes. Furthermore, AD GWAS signals, as well as the most strongly validated coding variants associated with AD (in APOE, TREM2 and ABCA7), coalesce around genes that are necessary for efficient phagocytic clearance of cellular debris by myeloid cells. Therefore, these loci represent excellent candidates as the first step in the cascade of molecular events that link genetic risk factors to the altered innate immune function that contributes to AD pathology. Indeed there is conflicting evidence as to the relative importance in AD pathogenesis of peripheral myeloid cells that subsequently enter the brain versus tissue resident myeloid cells such as microglia. Because of their shared ontology regulation of expression of many myeloid specific genes is likely to be shared between monocytes and microglia. Given the ease of access to blood monocytes throughout the disease process, compared to microglia that are only accessible at autopsy we propose to explore the functional consequences of commonly- occurring genetic variation on the transcriptome in peripheral monocytes from AD patients. Our central hypothesis is that peripheral monocyte-derived cells, such as macrophages and monocytes will manifest changes in gene expression of these AD susceptibility genes and other genes in the same molecular pathways that reflect the stages of AD pathophysiology. To test this hypothesis, we will characterize the transcriptome/methylome profiles from peripheral monocytes of 200 AD cases and 200 age-matched controls. This will be followed up with profiling of monocytes stimulated with anti- (myelin) and pro-inflammatory (LPS and A?) stimuli. Through innovative computational approaches, we will integrate various datasets from monocytes in order to identify causal drivers and molecular networks underlying AD pathogenesis such as A? clearance and neuroinflammation. We will also incorporate data from over 500 AD brains to assess if the monocyte-specific transcriptional networks recapitulate changes seen in the AD brains. Finally, we will perform highly multiplexed mass cytometry-based immune phenotype profiling to investigate the activation state and phagocytic capacity of monocytes. We will validate our most promising candidate genes from the functional studies in human microglia using immunohistochemistry. The proposed research is innovative because it will not only identify genetic mechanisms in peripheral monocytes which may contribute to interindividual risk for AD but may lead to the discovery of novel immune biomarkers.